Chronic Traumatic Encephalopahy (CTE) is a complex, evolving area of neurology and sports medicine. In one corner we have experts who definitively state that CTE is a delayed and progressive neurodegenerative disease directly linked to repeated head impacts with onset of symptoms later in life; while in the other there are those who suggest the evidence is currently insufficient to provide such a definitive statement.
High levels of post-mortem histology neurodegenerative change where first described in boxers in 1928 and termed ‘dementia pugilistica’ which, by 1949 would give rise to the term chronic traumatic encephalopathy. These neurodegenerative changes visible in the brain at autopsy are thought to occur from repetitive head impacts and include axonal disruption/injury, myelin degeneration, white matter loss, the presence of p-tau proteins (p-tau pathology), neurofibrillary tangles (NFT), astrocytic tangles (AT), beta-amyloid plaques (Aβ) among others. The distribution of these deposits is in clusters around small blood vessels of the cortex, typically at the sulci depths. Such changes are proposed to result in mood, behaviour, cognitive changes and dementia. Theories regarding the pathogenesis include disruption to the blood brain-barrier resulting in ischaemia within the brain and the presence of neuroinflammation following head impacts.
Head impacts such as sports related concussion can be disturbing to witness; the acute, delayed and prolonged symptoms can be distressing and challenging for an individual to work through. This has created space for the media to leverage clicks and views off through movies and stories using the narrative that sport related concussion is directly linked to CTE.
The first assumption in such a narrative this is that we definitively know that sport related concussion is exclusively a head impact injury, where symptoms are directly associated with injury to certain areas of the brain. For if this statement were not true how could we confidently link repeated head impacts to long term neurodegenerative change?
Our current understanding of sport related concussion is improving however, it is based on animal and motor vehicle occupant studies which are not representative of sports related concussion; head impact sensors are not as consistently accurate as required and, although blood biomarkers look increasingly promising their presence is nowhere near definitive. Furthermore, many symptoms of sport related concussion may not be directly associated with the brain and can be seen in whiplash patients leading to the plausible theory that sport related concussion may potentially include involvement of the upper cervical spine. There is no doubting that direct impacts to the head, such as a head-to-head contact results in a brain injury however, our understanding and ability to accurately diagnose sports related concussion is still limited and evolving.
The second assumption is that we can definitively identify the symptoms of CTE to make an accurate diagnosis. Presently there is no way to diagnose a living person with CTE, all diagnoses have been made via autopsy. Although recently there have been attempts to establish research diagnostic criteria for diagnosing traumatic encephalopathy syndrome, the clinical disorder suggestive of CTE. Moreover, there have been a series of pathohistological diagnostic criteria put forth by Omalu in 2011 which were broad and non-specific for CTE, nor did it include specific characteristics of p-tau pathology, a key feature of CTE; McKee in 2013 was based on a comparison study of 68 cases aged match to 18 control cases without a known history of mild traumatic head injuries and although they identified unique patterns and distributions of tau pathology in CTE the criteria has not been statistically tested, there are blurring between stages, as well as overlaps with other comorbid conditions. In 2016, a consensus panel of neuropathologists established a preliminary criteria for diagnosing CTE which as they stated, is limited and requires further research; additionally the methodology they used was at risk of selection bias. Furthermore, a key issue at present is that many of the histopathological findings seen in those with suspected CTE can also be present in other neurodegenerative conditions, and have been present in those who have not participated in contact sports.
This is not to say that neurodegenerative changes do not occur in those exposed to repetitive head trauma; there is evidence that shows athletes involved in contact sports have a higher risk of developing neurodegenerative diseases such as Dementia, Alzheimer’s, Parkinson’s and Motor Neuron Disease. Furthermore, the consensus panel found potentially distinctive p-tau pathologies unique to individuals with CTE. However, there is still much that needs to be learned about sports related concussion and CTE, as well as the relationship between the two. Currently, there are too many unanswered questions to support a definitive link between sports related concussion and CTE. Further research including longitudinal studies are needed to develop and evaluate blood biomarkers and imaging to help identify, diagnose and monitor sports related concussion and CTE, as well as determining what histopathological findings are specific to CTE, is CTE a specific disease entity or is it related to other neurodegenerative diseases like Alzheimer’s disease and is there a dose-effect, ie. a number or size of head impacts that are required before neurodegenerative change occurs.
The next time a headline splashes across the news linking contact sports to long term neurodegenerative changes in the brain, it is important that we take a step back for a moment and appreciate that our awareness and ability to identify those who have had a suspected sport related concussion has improved greatly, with immediate removal from play, and no return to play on the same day rules in place; sport governing bodies are continuously evolving rules to protect the heads of athletes; although more work needs to be done in junior sport, awareness among the community is improving. Additionally, we need to be look at ways we can encourage people to participate in sports safely, as opposed to frightening people off sport, with evidence supporting longer life expectancies in elite athletes including those in team sports. As we can continue to advance how we protect our athletes’ heads, we further mitigate the risk of any longer term changes that may occur in the brain.
With thanks to Dr. Jon Patricios